Authorization in asp.net core is simply AuthorizeAttribute

[Authorize]
public class SomeController : Controller
{
    public IActionResult Get()
    {
    }

    public IActionResult Post()
    {
    }
}

This will only allow a logged in user to access these actions.

or use the following to only limit a single action

public class SomeController : Controller
{
    public IActionResult Get()
    {
    }

    [Authorize]
    public IActionResult Post()
    {
    }
}

If you want to allow all users to access one of the actions you can use AllowAnonymousAttribute

[Authorize]
public class SomeController: Controller
{
    public IActionResult Get()
    {
    }
    
    [AllowAnonymous]
    public IActionResult Post()
    {
    }
}

Now Post can be accessed by any user. AllowAnonymous always comes as a priority to authorize, so if a controller is set to AllowAnonymous then all it's actions are public, regardless of if they have an AuthorizeAttribute or not.

There is an option to set all controllers to require authorized requests -

services.AddMvc(config =>
{
    var policy = new AuthorizationPolicyBuilder()
        .RequireAuthenticatedUser()
        .Build();
    config.Filters.Add(new AuthorizeFilter(policy));
}) 

This is done by adding a default authorization policy to each controller - any Authorize/AllowAnonymous Attributes over a controller/action will override these settings.